The present invention pertains to pipe venting systems, and in particular to a venting system specially suited for use in conjunction with plastic lined pipe.
Lined pipe is often utilized in handling of fluid materials having a corrosive nature. Lined pipe generally consists of a rigid outer casing and an inner corrosive resistant lining. Although the synthetic lining materials typically used are highly resistant to corrosion, they are susceptible to permeation by certain gases. Permeated gases then become trapped between the liner and the casing and can, if not properly vented, cause the liner to distort and separate from the casing, and at times cause its complete collapse or rupture.
The most conventional method of venting permeated gases is to drill a series of vent holes in the casing along its length. In order to avoid weakening of the casing, the holes formed are of a relatively small size and few in number. The small diameter holes, however, often become plugged by subsequent sandblasting and painting operations, which in turn frustrate proper venting. To avoid such plugging, the vent holes must be cleaned. Such cleaning is usually performed in the field. In addition, the holes can be covered with insulation and thus require that special vent extender tubes be added to the pipe. The vent extender tubes add extra cost to the pipe system. When field fabrication is required, the holes must be added to the pipe at the site. Such field work greatly increases the labor costs and time of installation, since the fabricator must (before installation) remove the liner, drill and deburr the holes, and then push the liner back into place.
A second manner by which pipe venting has been accomplished is through the use of a locking collar positioned at the juncture of two lined pipes. The collar is not designed for venting, but rather for locking the face of the liner against movement (a phenomenon commonly known as creep). Venting occurs because the metal locking collar and pipe flange face do not form a perfect seal. Although the gases are often able to pass through some tortuous path at the interface, this system is fraught with uncertainty since each interface will be substantially different and the very small paths are susceptible to plugging and thus fouling of the venting.
Further, with the use of either of the past venting systems, failure of the lining can cause an extremely hazardous and troublesome situation. More particularly, the paths through which venting occurs are random, unpredictable and completely open to the surrounding environment. Failure of the lining could, then, cause severe leakage into the surrounding environment and thereby create a health hazard, a danger to surrounding equipment and/or a situation involving a very labor-intensive and costly clean-up.
In an effort to control the venting of permeated gases, specially designed pipe casings and flanges have been developed which centralize the venting openings to facilitate containment thereof. An example of such a construction is disclosed in U.S. Pat. No. 3,506,039 issued Apr. 14, 1970 to Marriott, and entitled VENTING OF LINED PIPE. However, special machining of the pipe segments and flanges greatly increases the cost involved in manufacture.